Tubular mandrels are used for the manual shirring of such flexible tubular material as well as for the automatic shirring thereof by machines. These mandrels support the flexible tubular material while it is being shirred during the folding operation and to calibrate and thus form the corrugated tube prior to the later attachment of the tube to a stuffing nozzle of a sausage stuffing machine. By this arrangement the inwardly pointing folds of the corrugations sit snugly against the surface of the mandrel which, in the case of a shirring machine, makes difficult the subsequent stripping of the corrugated tube from the expander.